Abstract

River morphology and behaviour are components of overall landscape behaviour, which responds to tectonic, volcanic, climatic and anthropogenic inputs. River morphology and behaviour adjust in response to these inputs to allow the available flow to transport the sediment supplied to base level at the rate at which it is supplied. To be sustainable, management of rivers to reduce flood hazards must therefore include measures to maintain the natural sediment transport capacity of the river. This is difficult in principle; there is evidence that the natural state of a river corresponds to a local maximum of bedload sediment transport capacity. Hence any moderate alteration will cause a decrease in sediment transport capacity, leading to aggradation, steepening of gradient and bank erosion. To achieve an artificial increase in sediment transport capacity therefore requires a substantial alteration of river form (usually narrowing), which in turn requires substantial structural works to maintain flood discharge and control local erosion. Experience in both Japan and Switzerland suggests that modification of rivers for flood control can be sustainable in both high and low sediment input environments if the economic base is sufficient to justify large expenditure on substantial river engineering works. However, New Zealand experience indicates that modification of active bedload dominated rivers is much less sustainable in low- to moderately-developed land areas, where the required substantial modifications are not economically justifiable.